One method for processing semiconductor wafers to produce integrated circuits involves the use of 1:1 photolithography, wherein a photomask is used which has a full size image of the pattern to be transferred to the wafer. Exposure may be accomplished in a step and repeat process in which the photographic image is sequentially exposed on each of the individual cells of the wafer. It is conventional to refer to a photomask used in a step and repeat process as a "reticle".
Alternatively, the entire surface of a wafer may be exposed using a photomask with a full wafer image.
Both types of photomasks generally comprise a plate of quartz or low expansion glass with a thin layer of chromium deposited on one surface into which the image is etched. Regardless of the type of photomask used, cleanliness of the mask is critical to producing a wafer in which defects are minimized. Particulate matter on the etched surface of the photomask or anywhere within the focal length of the exposure lens will likely produce a defect in the processed wafer. To minimize such defects, it is common to cover the image area of the photomask with a dust cover. Such covers typically consist of a frame of aluminum or other suitable material having a thin transparent membrane of nitrocellulose or a similar material. The thickness of the frame is such that the dust cover will preclude particulate matter within the focal length of the exposure lens. Frames are typically approximately 0.050 inches thick. Such a dust cover is commonly referred to as a "pellicle". Pellicles are typically attached to a photomask with an adhesive coated vinyl tape having a thickness of 5 mils or greater.
Pellicles need to be applied only to the etched surface of the photomask since the thickness of the glass substrate precludes airborne particulates from coming within the focal length of the lens on the side of the photomask opposite the etched lamp.
Since the delicate membrane of a pellicle is easily damaged, it is frequently necessary to remove a pellicle from a photomask in order to apply a new pellicle. Mechanical removal by prying off the pellicle is frequently unsuccessful since the photomask is easily damaged.
A more common technique for pellicle removal is to place the photomask in an acetone bath to at least partially dissolve the adhesive. This technique may require from four hours to four days to detach a pellicle. Such a lengthy process cannot be tolerated if a production line is stopped awaiting rework of a photomask. Furthermore, this pellicle removal technique suffers the inherent disadvantage of using a hazardous solvent.
As will be subsequently described, the present invention provides a controlled mechanical means of removing a pellicle. Accurately positioned cutting blades are inserted between the photomask and the pellicle such that the pellicle may be removed from the photomask very quickly, without the use of hazardous chemicals and with minimal risk of damage to the photomask.